Here's a suggestion that doesn't require any liners at all.

Consider using plain old nursery pots inside the planters. This way, you can easily switch out old plants and switch in new ones as the seasons progress. You can use different size and height pots by standing shorter ones on bricks, and cover the tops of all with coir or sphagnum moss to hide the pot tops.

I made containers from 2" styrofoam sheets and painted them. Next spring I plan to wrap the whole system w/wood so the foam boxes will become liners. My neighbor used blue, building insulation foam for his. I like it better and will switch to that for the rest of my project because it's easier to work with. The styrofoam was a mess during fabrication. The blue stuff is readily available at any lumber yard. It's cheap, won't break down, and will keep the soil a little cooler. Don't forget drainage opennings in the bottom. Custom cutting the foam yourself (very easy w/break away blades) lets you use all the space in your planters.

I have found that wood planter boxes last a long time with a properly installed liner. I buy thin HDPE material from www.justliners.com and fold it up the in-sides and fasten it over the side boards under the top trim. For drainage I use a short piece of plastic water pipe to extend about 1/4 inch below the wood bottom. Pre-drill the bottom to fit the pipe. Heat and expand the top lip of the pipe so that it can't pass through the drilled hole. A ball peen hammer head works fine. Use a little building caulk under the top lip of the pipe and punch it through the liner. This avoids leakage and rot. I know this sounds like a lot of trouble, but when building planter boxes you may as well go the whole route so that you can enjoy them for many extra years.

Here is a link that might be useful: Just pond liners